HHighlights from PHENIX - I
A Franz (for the PHENIX Collaboration ‡ ) Brookhaven National Laboratory, Upton, NY 11973-5000 USAE-mail: [email protected]
Abstract.
This contribution highlights recent results from the PHENIX Collaboration atRHIC. It covers global variables, flow and 2–particle correlations. A secondcontribution in this issue, by T.C.Awes, covers PHENIX results on heavy quarks,leptons and photons.PACS numbers: 25.75.-q, 25.75.Ag, 25.75.Gz, 25.75.Ld
Submitted to:
J. Phys. G: Nucl. Phys.
1. Introduction
The Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory(BNL) in Upton, NY has just finished its 8 th year of operation. The PHENIXCollaboration with its to–date 476 scientists from 67 institutions and 14 Nations hascollected in the recent d–Au and p–p run a record 577 TB of data and 275 billionsevents. The Run–8 d–Au sample represents a 30 times increase over the Run–3 dataset despite the addition of new detectors which are described in Sect. 2 in more detail.These large data samples allow us to probe the properties of the new matter withprecision measurements of the distributions and systematic study of their dependence oncolliding system, centrality, rapidity or even the reaction plane. RHIC also increased itsluminosity by a better understanding of the machine and new techniques like stochasticcooling.RHIC is likely the most versatile heavy ion collider in the world and has collided inits first 8 years 4 different species at 6 different beam energies. Table 1 shows a summaryof these first 8 years of PHENIX data taking.The different collisions systems varying from simple p–p and d–Au, where coldnuclear effects should be visible, serving as a baseline and via proper scaling as acomparison to the collisions of heavier ions, e.g. Cu–Cu and Au–Au. These comparisonsshould enhance the difference of scaled p–p collisions to the properties of the produceddense medium. In 2003 all four RHIC experiments published white papers [1] to ‡ A list of members of the PHENIX Collaboration can be found at the end of this issue a r X i v : . [ nu c l - e x ] M a y ighlights from PHENIX - I Table 1.
Summary of the first 8 years of RHIC running for the PHENIX experiment
Year Species √ s [GeV ] (cid:82) Ldt N tot
Data Size(sampled)Run–1 2000 Au–Au 130 1 µ b −
10 M 3 TBRun–2 2001/02 Au–Au 200 24 µ b −
170 M 10 TBAu–Au 19 < − − − µ b − µ b −
58 M 10 TBRun–5 2005 Cu–Cu 200 3 nb − − µ b − −
85 B 262 TBRun–6 2006 p–p 200 10.7 pb −
233 B 310 TBp–p 62.4 0.1 pb −
28 B 25 TBRun–7 2007 Au–Au 200 813 µ b − −
160 B 437 TBp–p 200 5.2 pb −
115 B 118 TBAu–Au 9.2 few ksummarize their findings which led to the announcement that a new phase of matterhad been found [2].
2. New PHENIX detector subsystems
Figure 1 and 2 show the PHENIX detector in the 2007/2008 configuration. It consistsof 4 spectrometer arms with 3 main magnets. Two arms at mid-rapidity (East andWest) with tracking, particle identification (PID) detectors and calorimeters for hadron,electron and photon detection and two muon arms in the forward angles (North andSouth). Details can be found in [3].Several new detectors had been added over the past years to improve PID, themeasurement of the reaction plane (RP) and π identification at forward angles.A time-of-flight (TOF-W) detector, based on multi-gap resistive plate chambers(MRPC) [4], was added to the PHENIX West central arm detector in 2007 to extendthe PID to higher momenta, i.e. above 2-3 GeV/c. Before the PID in the PHENIX Westarm relied on a combination of a gas Ring Image Cherenkov (RICH) vessel, an aerogeldetector (n=1.0114), and the electromagnetic calorimeter (EMCal) which left a gap inthe pion to kaon separation between 3 and 5 GeV/c. One octant of these MRPCs with ighlights from PHENIX - I West
Beam View
PHENIX Detector 2008
East
MPC RxNPPbSc PbSc PbSc PbSc PbSc PbGl PbSc PbGl TOF-EPC1 PC1 PC3 PC2 Central Magnet TEC PC3 BB RICH RICH DC DC AerogelTOF-W
Figure 1. View of thePHENIX detector inbeam direction.
South
Side View
PHENIX Detector 2008
North
MuTrMuID MuIDRxNPCentral Magnet N o r t h M u o n M a g n e t S ou t h M uon M a gn e t BBCMPC ZDC NorthZDC South
Figure 2. View of the PHENIXdetector in side–view. pad readout were installed and achieving a 75ps time resolution, 85ps overall with theBeam–Beam Counter (BBC), the interaction and TOF start detector, resolution foldedin. Several new results based on this detector have been presented at this conference[5, 6].For a few years PHENIX will use the new Reaction Plane Detector (RxNP) [7] tofurther improve the RP measurement and to improve triggering at lower energies whenthe BBC and Zero–Degree–Calorimeters (ZDC) are not efficient enough. The RxNPconsists of 2x2 rings with 12 scintillator counters each, read out by 2x24 photomultipliers.It covers the pseudorapidity windows η = 1 . → . , . → . crystals into the forward tips, 3 . < | η | < .
7, of each magnetpiston in the North and South muon magnets. The main goal for the Muon PistonCalorimeter (MPC) [8], as it is called, is the reconstruction of π and the search for spinasymmetries in p–p collisions. In heavy ion running, when the overall multiplicity is toohigh, it improves the measurement of the event reaction plane.A detector which had a first engineering run in 2007 is the Hadron Blind Detector(HBD) [9, 10]. Its a windowless Cherenkov detector using pure CF with a triple GEMreadout, where the top most layer is coated with Cesium Iodide (CsI) to convert theCherenkov photons into photo-electrons which are in turn amplified by the GEM witha gain of ∼ · . The HBD will be important in coming years for the measurement oflow mass electron pairs from the decay of light vector mesons ( ρ , ω , and φ ).In the coming years PHENIX plans to install a silicon vertex tracking system, see[11], a forward silicon–tungsten calorimeter, and a muon trigger based on resistive platechambers. ighlights from PHENIX - I
3. Global Observables
Presentations at Quark Matter 1987 in Nordkirchen [12], more than 20 years ago,concentrated on the first measurements of global observables like event multiplicity,transverse momentum and energy distributions to understand if the levels of energydensities reached where sufficient to form a QGP. More than ten years later, in 2000,further detailed measurements lead to the announcement [13] that a new state of matterhad been observed.
Figure 3. Transverse energyversus number of participatingnucleons in Au - Au collisions. Figure 4. Transverse energyversus √ s for central Au - Aucollisions. PHENIX had published measurements of the total transverse energy, E T , for √ s = 200, 130 and 13.9 GeV/c previously [14, 15]. Figure 3 summarizes thesemeasurements for Au–Au collisions as a function of participating nucleons and addsthe distribution for the fourth beam energy. E T increases with increasing number ofparticipants and stronger with higher beam energy. Concentrating on the most centralcollisions, Figure 4 shows dE T / ( dη . N p ) as a function of √ s for several measurementsincluding the new PHENIX datapoint at √ s = 62.4 GeV/c. The measurement falls wellin line with the previous observed linear dependence of the scaled E T with the log of √ s . Detailed studies of charged particle multiplicities in smaller and smaller rapiditywindows are accessible with the large datasets. In this volume PHENIX presentsfluctuation studies in overall multiplicities and particle ratios [16]. Deviations from amonotonic behavior in these ratios should indicate a possible phase transition or critical ighlights from PHENIX - I B , for deuterons [20, 6]. Using the above mentioned TOF-W detector PHENIX couldextend the existing (anti–) deuteron measurements to higher p T and multiple centralitybins. Expressing the coalescence probability, B − is a measure of the source radius. Itincreases linearly with the number of participating nucleons in the collision, and theextracted radius parameter is compatible with HBT results on pion pairs.
4. Flow
A most surprising observation at RHIC was the strong elliptic flow, which lead to theconclusion that the medium we are studying does not behave like a hot gas but ratherlike a strongly coupled liquid. When colliding at intermediate impact parameters theoverlap region between the two nuclei is elliptically shaped in the transverse plane.This spacial anisotropy creates a pressure gradient which translates into a momentumanisotropy in the final particle stage. Experimentally this is measured via the φ angulardistribution of the particles with respect to the reaction plane angle, Ψ R , of the eventwhich is defined by the beam direction and the distance vector of the center of the twonuclei [21], and a Fourier decomposition. E d Ndp = 12 π dNp T dp T dy (cid:34) ∞ (cid:88) n =1 v n ( p T , y ) cos( nφ ) (cid:35) (1)Because of the symmetry φ ↔ − φ in the collision geometry, sine terms do not appear inabove expansion. Also the odd-order anisotropic flows of particles at midrapidity vanishin collisions with equal mass nuclei as a result of the additional symmetry φ ↔ φ + π .The second coefficient in the Fourier transform, v , is usually the largest and hasbeen studied by all RHIC experiments. It has been observed that the v of all studiedparticles scales as v /n q ∼ KE T /n q , where n q is the number of quarks in the particleand KE T = m T − m is the transverse kinetic energy. This scaling was observed up to KE T ≈ GeV /c , an indication that hydrodynamical description of the data was valid[22]. New data from PHENIX [5] indicate that this is not valid above 1 GeV/c whichcorresponds to p T ≈ GeV /c for a proton, the region where hard scattering becomesimportant.The next higher term v is an important measure if a ideal hydrodynamicaldescription is applicable in this momentum range. If valid, v should follow the samescaling in KE T as v , but scaled with n q and more important be equivalent to v n − q asdemonstrated in the right panel of Figure 5. ighlights from PHENIX - I Figure 5. v as a function of p T , KE T and KE T /n q (GeV/c) T p v -0.3-0.2-0.100.10.20.3 Coalescence at freeze-out (PLB595,202)in transport model (PLB655,126)in fireball (PRL97:232301)+ initial mix (Zhao, Rapp priv. comm.) (PRL97:232301) ! Initially produced J/Comovers (Lynnik priv. comm.)
PHENIX |y|<0.35 [20,60%] (Preliminary) 3% ± Global Syst. [0,5] GeV/c " T p 0.02 ± ± = -0.10 v
42% of Run-7
Figure 6. v for J/ Ψ → e + e − measured at mid-radidity in Au – Au collisions with thePHENIX central arm detectors. The lines indicate theoretical predictions as indicatedin the figure. Figure 6 shows the first data on the v of J/ Ψ → e + e − in heavy ion collisions. Thepreliminary result of v = − . ± . ± .
02 is compatible with 0, but only 42% ofthe data are analyzed so far. The lines in Figure 6 represent theoretical predictions asindicated. A PHENIX result on J/ Ψ → µ + µ − is to be presented soon, see [23] for moredetails. ighlights from PHENIX - I v starts to diverge from the RP v at p T ≈ . GeV /c , indicating that non–flow effects, e.g. jets from hard scattering, become important. Incidentally it is also theregion where the KE T /n q scaling starts to fail.
5. Jets
PHENIX has studied the properties of jets at 200 and 62.4 GeV/c for p–p, d–Au, Cu–Cu,and Au–Au collisions [26, 27, 28, 29, 30, 31, 32].
Figure 7. Near (squares) and awayside (circles) transverse momentumspectra for Au–Au (filled) and p–p (open) collisions for different rapidityregions for the near and away side jets. The lines represent fits tothe datapoints, with the solid line indicating the fit to an inclusive p T distribution. Jets resulting from a hard scattering of partons are impossible to reconstruct in ighlights from PHENIX - I p T particle, assumed to be the leadingparticle of one jet–arm and all other particles assumed to be from the same jet or therecoil. The correlation functions have to be corrected for background and flow, whichin itself is an angular correlation.It has been observed in p–p and peripheral A–A collisions that opposite the triggerparticle jet (near side) a slightly wide correlated distribution emerges (away side). Incentral A–A collisions the momentum spectra for the away side softens and the angulardistribution widens even more. Several explanations for these effects have been presentedat this conference. Figure 7 shows a PHENIX comparison of the momentum distributionfor two pseudo-rapidity, η , regions of the near and away side in p–p and Au–Au collisions.The momentum spectra for Au–Au collisions on the near side, but away from the mainjet (upper, blue, solid points), and the away side (lower, red, solid points) are softercompared to p–p (open points) and close to the inclusive spectra (lines). This indicatesthat the momentum distributions of these particles has been softened by passing throughthe medium.If the particle distributions and momenta are affected by their passage through thecollision medium than the distributions when measured along the long versus the shortaxis of the collisions ellipsoid should be different. Figure 8 shows preliminary PHENIXresults on a 2 particle correlation function were the data are binned in angular regionswith respect to the reaction plane. Even so the v dominated systematic error is largea clear change in the shape of the distributions is visible. Figure 8. Jet correlations functions for Au Au collisions, panels represent15 degree slices from 0 - 90 degrees away from the reaction plane. ighlights from PHENIX - I
6. Summary
PHENIX has collected a vast sample of data from p–p to Au–Au collisions at variousenergies. The data show that we have created a dense medium which affects themomenta and angular distributions of the produced particles. On the other hand itshows a strongly coupled flow which affects all produced particles, even heavy quarks.PHENIX has shown multiple new and more detailed results at this conference and willwith its current and future detector subsystems continue to uncover the details of this’perfect liquid’.
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